This invention claims priority of German Patent Application DE 101 12 611.5, the entirety of which is incorporated herein by reference, filed Mar. 14, 2001.
1. Field of the Invention
The invention relates to a variable viewfinder for wide angle shots.
2. Description of the Related Art
An optical system for a variable viewfinder is described in U.S. Pat. No. 4,725,130 (the xe2x80x9c""130 patentxe2x80x9d) which is incorporated herein by reference. The ""130 patent is particularly concerned with compact cameras including photographic cameras and video cameras. These cameras are frequently fitted with zoom lenses that are designed for a relatively limited focal range. The viewfinder magnification is adjusted, by gearing, as a function of the change in the focal length under cam control so that the image field in the viewfinder corresponds to the image field for the shot. Two of the lens units are adjusted relative to a fixed lens unit to adapt the viewfinder magnification. In this process, at least one of the outer lens units is also adjusted so that the overall length of the viewfinder system is changed.
Cameras with a measuring viewfinder and interchangeable lenses of different focal length are usually equipped with a viewfinder of constant magnification that is optimized for a selected lens focal length. When using a lens having a longer (or shorter) focal length, various frames corresponding to the changed image field of the shots are reflected into the viewfinder. As the focal length of the lens increases, the image field, limited by the frame, decreases. Correspondingly, as the focal length decreases, the image field increases until it can no longer be displayed in the viewfinder; the user can then no longer fully observe in the viewfinder the image field taken by the lens.
To remedy this defect, special viewfinders are provided for wide angle lenses. The image fields of wide angle lenses are adapted to the lens and can be pushed, via an adaptor, into the flash shoe on the camera. The basic design of such systems, which are known as Galilean viewfinders, constitutes an inverted Dutch telescope. This design has a divergent lens on the object side which limits the field of view and, therefore, frequently has a shape similar to the picture size; additionally, a mask corresponding to the picture size may be added. A convergent lens acts as an eyepiece. The field of view is a function of the mask size and the ratio of the focal lengths of the two lenses.
The fact that the user has to set the distance, stop, and look into the viewfinder on the camera side results in particular design requirements for the push-on viewfinder. Looking into the camera viewfinder is not to be prevented by the push-on viewfinder. The distance, when switching between looking and not looking into the push-on viewfinder, should not be too large. For the viewfinding magnification to be as large as possible (so as to be able to detect detail effectively), the diameters of the lenses in the front lens unit must be correspondingly large. On the other hand, high speed wide angle lenses have a large diameter which can impede the field of view of the push-on viewfinder when the latter is arranged as close as possible to the camera housing.
As a result of these design-induced stipulations, only special push-on viewfinders have been developed as Galilean viewfinders of simplest design for specific focal lengths. Historically, this was tolerable for the user because the magnification selected for the built-in viewfinder permited the display of a viewfinder frame for lenses with a focal length of up to, for example, 28 mm. Moreover, use was mostly made only of one further lens of shorter focal length. However, because of better setting accuracy for standard lenses, there is an increasing desire for higher viewfinder magnifications. As a result, the field of view can no longer be displayed for, by way of example, a 28 mm lens. Moreover, owing to modern methods of computation and manufacture, the design of wide angle lenses with shorter focal lengths and higher imaging performance in conjunction with high speed is something which is finding increasing favor among users. However, to achieve such desires, users must use an ever greater number of push-on viewfinders, which increases the likelihood of confusion in assignment when exchanging lenses and viewfinders.
For these and other reasons, a new apparatus is needed which provides a variable viewfinder which is easy to assemble and attach to a camera and which provides an increased magnification range without increasing the overall size of the viewfinder.
A first aspect of the invention involves a viewfinder having a variable magnification. The viewfinder includes a telescope housing, a rotating ring, a variator, and first, second, and third lens units. The telescope housing, which may be a cylinder, has an axis which defines a light direction. In addition, the telescope housing includes at least one curved groove formed in a sidewall thereof and an adapter. The rotating ring, which is rotatably connected to the telescope housing, includes at least one longitudinal groove which is substantially parallel to the axis. Similarly, the variator, which is preferably made out of an elastomeric material such as injection-molded plastic, is journalled within the telescope housing and is adapted to move within the telescope housing in the direction of the axis. The variator also includes a plurality of guide pins at least one of which projects through one of the curved grooves and extends into one of the one longitudinal grooves.
The first and third lens units have negative refractive power in the light direction whereas the second lens unit has positive refractive power in that direction. In addition, all three lens units are positioned in the telescope housing, however, only the first and third lens units are fixed relative thereto. The second lens unit, on the other hand, is provided in the variator and is adapted to move with respect to the first and third lens units along the longitudinal axis to change the magnification of the viewfinder. In addition, the adapter is adapted to fit in a shoe of a camera.
In the aforementioned viewfinder, the first lens unit may include a first and a second lens between which is provided a mask which is adapted to limit an image field of the viewfinder. Moreover, the mask may include a position-determining nose that is adapted to engage a cutout in the telescope housing. In addition, the second lens of the first lens unit may rest on a snap ring which is inserted in a circumferential groove in the telescope housing.
The telescope housing is preferably open at one end; the open end being adapted to receive light traveling along the axis. Moreover, the open end may include a holder to maintain the orientation of third lens unit with respect to the telescope housing. Similarly, a holder, which may be provided on a head side of the variator, may maintain the orientation of the second lens unit with respect to the variator. Moreover, a rear side of the variator may include at least one tongue from which the guide pins project in a direction radially away from the axis. Further, the guide pins and the tongues are preferably integrally formed. In addition, sliding surfaces may be provided on an outer surface of the variator.
The adapter preferably includes a ball latch which is adapted to engage releaseably latching notches in the rotating ring. In addition, the telescope housing preferably includes a holder adapted to receive a supplementary lens.
In the previously described viewfinder, the focal lengths of the three lens units are such that the diopter of the viewfinder, over a focusing range, lies in a range that a human eye can accommodate. Moreover, the viewfinder diopter, over the focusing range, is preferably between xe2x88x920.5 and xe2x88x921.0.
These and other features, aspects, and advantages of the present invention will become more apparent from the following description, appended claims, and accompanying exemplary embodiments shown in the drawings.